Method for producing aluminum alloy having improved semi-solid molding capability and billet thereof

ABSTRACT

A method for producing a billet of an aluminum alloy having an improved semi-solid molding capability, in which the aluminum alloy comprises, in wt %, 0.005 to 0.5 Ti, 0.0001 to 0.1 B, and the balance substantially aluminum, comprises pouring the aluminum alloy in a molten state into a cooling mold to perform the semi-continuous casting of the aluminum alloy; providing a billet having a solidified layer on its surface by primary cooling-down of the molten aluminum alloy using the mold; pulling the billet out of the mold; and feeding a refrigerant onto the surface of the billet to effect secondary cooling-down of the billet. The primary cooling-down is separated from the secondary cooling-down by the distance of 150 mm or less. The refrigerant for the secondary cooling-down is in contact with the billet at an angle between 20° or greater and less than 80° in the casting direction.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to a method for producing an aluminum alloy having an improved semi-solid molding capability, and a billet of the aluminum alloy.

2. Description of the Related Art

Thixocasting based on a semi-solid, molded billet is an art having recently attracted considerable attention because of its advantages. The advantages include reductions in internal defects such as casting macro segregation, gas inclusion, and shrinkage cavities, when compared with conventional die-casting processes. The advantages further include improved mechanical characteristics, improved dimensional accuracy, and a longer service life of a mold, when compared with conventional die-casting processes.

To cast the billet for use in the Thixocasting, system “A” such as a method referred to as the Pechiney or Alumax System, has widely been known, and has already been in practical use. According to system “A”, a melt is electromagnetically stirred at a semi-solid temperature range to provide a spheroidized primary α (Al) phase at a billet-producing stage.

Another system “B” is disclosed in JP Patent No. 3216684. According to system “B”, an aluminum alloy is poured into a mold only within the range of temperatures at most 30° C. above a liquidus line. The poured aluminum alloy is solidified and cooled down at a cooling rate of 1° C. or greater per second in a solidification zone. The solidified aluminum alloy is heated at the rate of 0.5° C. or greater per minute between the temperatures of solubility and solidus lines. The heated aluminum alloy is further heated up to the range of temperatures beyond the solidus line. Then, the aluminum alloy is as such retained for five to sixty minutes to provide spheroidized primary grains. Thereafter, the aluminum alloy is again heated up to a molding temperature below the liquidus line, thereby providing a semi-solid aluminum alloy. In this way, the semi-solid aluminum alloy is molded. The aluminum alloy has Ti and B added thereto.

SUMMARY OF THE INVENTION

However, system “A” has problems of a very complicated manufacturing process and expensive facilities, with a consequential increase in manufacturing cost.

System “B” involves the addition of a large quantity of Al—Ti—B, and TiB₂ settles down in a holding furnace, with consequential instability of casting quality. A further problem with system “B” is that complicated temperature control must be executed because the aluminum alloy is cooled down and heated up at temperatures near the liquidus line. A yet further problem with system “B” is that neither mass production nor a cost cutback is achievable because of complicated working processes.

In view of the above, the present invention has been devised to realize objects of the present invention. One of the objects of the present invention is to provide a method for producing an aluminum alloy having an improved semi-solid molding capability, and a billet of the aluminum alloy, in order to overcome the shortcomings of the prior art, to achieve a simpler manufacturing process and lower cost, and to provide homogenous microstructure and high-quality molded products. In particular, another object of the present invention is to perform initial operations during the semi-continuous casting of an aluminum alloy billet to be semi-solid molded, and to perform a subsequent operation during the casting of the aluminum alloy billet. The initial operations include the steps of: restricting a method for adding Ti and b to be added as a grain refiner; properly dispensing amounts of Ti and B to be added; and limiting the shape, size, and composition ratio of TiB₂. The subsequent operation includes the step of adjusting control over casting conditions to provide a fine grain structure of a billet.

To achieve the above objects, a first aspect of the present invention provides an aluminum alloy having an improved semi-solid molding capability, comprising, in wt %, 0.005 to 0.5 Ti, 0.0001 to 0.1 B, and the balance substantially aluminum.

According to the first aspect of the present invention, the aluminum alloy contains proper amounts of Ti and B, and can be formed by fine grain structure.

To achieve the above objects, a second aspect of the present invention provides an aluminum alloy having an improved semi-solid molding capability, as defined in the first aspect of the present invention, in which Ti to be added to provide fine grain structure of the aluminum alloy is added in the form of an Al—Ti master alloy before being further added in the form of an Al—Ti—B master alloy.

According to the second aspect of the present invention, Ti is initially added to the aluminum alloy in the form of the Al—Ti master alloy, and is then added thereto in the form of the Al—Ti—B master alloy. As a result, an aluminum alloy having a fine grain structure is attainable.

To achieve the above objects, a third aspect of the present invention provides an aluminum alloy having an improved semi-solid molding capability, as defined in the first or second aspect of the present invention, in which amounts of Ti and B to be added to provide fine grain structure of the aluminum alloy are in a Ti-to-B-ratio of 3 to 40, (Al_(x).Ti_(y)) B₂ (x=1 to 7, y=1 to 9) is a particle having the size of 10 μm or less, and an atomic weight ratio of Al_(x) to Ti_(y) is 0.2 to 2.5.

To achieve the above objects, a fourth aspect of the present invention provides a method for producing a billet of an aluminum alloy having an improved semi-solid molding capability, in which the aluminum alloy comprises, in wt %, 0.005 to 0.5 Ti, 0.0001 to 0.1 B, and the balance substantially aluminum. The method comprises the steps of: pouring the aluminum alloy in a molten state into a cylindrical, forced cooling mold from the top of the mold in order to perform the semi-continuous casting of the aluminum alloy; providing a billet having a solidified layer formed on the surface of the billet by executing the primary cooling-down of the molten aluminum alloy using the mold; pulling the billet out of the mold by lowering a bottom block which supports a bottom end of the billet; and feeding a refrigerant onto the surface of the billet to effect the secondary cooling-down of the billet. The first cooling-down is separated from the second cooling-down by the distance of 150 mm or less. The refrigerant for use in the secondary cooling-down is brought into contact with the billet at an angle between 20° or greater and less than 80° in the casting direction.

To achieve the above objects, a fifth aspect of the present invention provides a method for producing a billet of an aluminum alloy having an improved semi-solid molding capability, as defined in the fourth aspect of the present invention, in which Ti in the molten aluminum alloy is added in the form of an Al—Ti master alloy before being further added in the form of an Al—Ti—B master alloy.

To achieve the above objects, a sixth aspect of the present invention provides a method for producing a billet of an aluminum alloy having an improved semi-solid molding capability, as defined in the fourth or fifth aspect of the present invention, in which amounts of Ti and B in the aluminum alloy are in a Ti-to-B-ratio of 3 to 40, (Al_(x).Ti_(y)) B₂ (x=1 to 7, y=1 to 9) is a particle having the size of 10 μm or less, and an atomic weight ratio of Al_(x) to Ti_(y) is 0.2 to 2.5.

To achieve the above objects, a seventh aspect of the present invention provides a method for producing a billet of an aluminum alloy having an improved semi-solid molding capability, as defined in the fourth, fifth, or sixth aspect of the present invention, in which after the secondary cooling-off to enhance the semi-continuous casting of the aluminum alloy, a tertiary cooling-down refrigerant is brought into contact with the billet at an angle between 20° or greater and less than 80° in the casting direction to further increase the cooling-down of the billet.

To achieve the above objects, an eighth aspect of the present invention provides a method for producing a billet of an aluminum alloy having an improved semi-solid molding capability, as defined in the fourth, fifth, sixth, or seventh aspect of the present invention, in which the step of providing the billet having the solidified layer formed on the surface of the billet comprises the step of expelling the molten aluminum alloy from a float at an angle between 20° or greater and less than 80° in the casting direction to feed the molten aluminum alloy out of the mold in order to control the level of the molten aluminum alloy. The float is positioned in the mold on the surface of the molten aluminum alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a microscopic structure photograph illustrating a representative example of one primary α (Al) phase having fine uniformity assessed as good (◯).

FIG. 2 is a microscopic structure photograph illustrating a representative example of another primary α (Al) phase having fine uniformity evaluated as bad (X).

FIG. 3 is a cross-sectional view of a device performing a method of the present invention, in which a bottom block is near a mold.

FIG. 4 is a cross-sectional view of a device performing a method of the present invention, in which a bottom block is away from a mold.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a more complete understanding of the present invention, detailed descriptions will be made as to reasons for various numeric limitations such as numeric limitations in constituent amounts of an aluminum alloy according to the present invention.

Ingredient Ti provides a refined cast structure, and prevents billet interval cracking. However, Ti in the amount less than 0.005 wt % is less operative. Ti in the amount greater than 0.5 wt % encourages the occurrence of TiAl₃, huge intermetallic compounds. Therefore, according to the present embodiment, Ti having the content limited to 0.005 to 0.5 wt % was used.

Similarly to ingredient Ti, ingredient B is operative to realize a fine grain structure, and to avoid cracking a billet. However, B in the amount less than 0.0001 wt % is indistinctively operative. The addition of Ti in the amount greater than 0.1 wt % is unexpected to bring the beneficial effects commensurate with the added amount of Ti. Therefore, according to the present embodiment, B having the content limited to 0.0001 to 0.1 wt % was used.

What is important for Ti to be added to aluminum to provide fine grain structure of the billet is to initially add Ti in the form of an Al—Ti master alloy. When a predetermined amount of Ti is added in the form of an Al—Ti—B master alloy at a time, then TiB₂ in the master alloy settles down in the molten alloy, with a consequential failure in grain refinement commensurate with the added amount of Ti. Accordingly, Ti is initially added in the form of the Al—Ti master alloy to generate the fine grains before the addition of the Al—Ti—B master alloy, thereby providing a microscopic cast structure. Therefore, according to the present embodiment, a process for adding a grain refiner to provide a fine grain structure was practiced by initially adding Ti in the form of the Al—Ti master alloy and subsequently by adding a proper amount of Ti in the form of the Al—Ti—B master alloy.

Respective amounts of Ti and B to be added to aluminum to obtain the fine grain structure of the billet are determined by an optimum ratio of Ti to B. A Ti-to-B ratio less than 3 is insufficient to provide the fine grain structure. A Ti-to-B ratio greater than 40 saturates the grain refining efficiency. Therefore, according to the present embodiment, a Ti-to-B ratio falling within the range of 3 to 40 was applied.

The following discusses the compound (Al_(x).Ti_(y)) B₂ to be added to aluminum as well. The size of (Al_(x).Ti_(y)) B₂ influences the miniaturization of the grains to give smaller grain size. More specifically, spherical diameter of (Al_(x).Ti_(y)) B₂ greater than 10 μm dramatically diminishes the grain refining efficiency. In addition, (Al_(x).Ti_(y)) B₂ which are not particles diminish grain refining efficiency because (Al_(x).Ti_(y)) B₂ are to be the nucleus of the grains. Therefore, according to the present embodiment, the particle size of (Al_(x).Ti_(y)) B₂ is 10 μm or less.

According to the present embodiment, “x” and “y” in (Al_(x).Ti_(y)) B₂ were replaced by one set of 1 to 7 and another of 1 to 9, respectively. This is because “x” and “y” falling out of the respective ranges as discussed above reduce the effect of obtaining the fine grain structure of the billet.

A ratio between respective atomic weights of Al_(x) and Ti_(y) which are parts of the particle size of compound (Al_(x).Ti_(y)) B₂, materially contributes to the refinement of grain structure. When the atomic weight ratio of Al_(x) to Ti_(y) is less than 0.2, then the grain refining efficiency is reduced. The grain refining efficiency is diminished as well when the atomic weight ratio of Al_(x) to Ti_(y) is greater than 2.5. Therefore, according to the present embodiment, Al_(x) and Ti_(y) in the ratio of 0.2 to 2.5 were used.

The fine grain structure of the billet is obtained by the addition of the grain refiner, and it is also obtained by rapid cooling of the molten aluminum alloy to increase solidification rate of the molten aluminum alloy and to restrain grain growth in the aluminum alloy.

A method of the present invention will be detailed below, referring to FIG. 3 and FIG. 4.

To practice the semi-continuous casting of the aluminum alloy, the present embodiment employs a casting process using a cooled mold 5 having the top and bottom opened, in which the cooled mold has a bottom block 8 disposed to close the open bottom of the mold 5. The above casting process according to the present embodiment includes the steps of: introducing the molten aluminum alloy 1 in a trough 2 into the mold 5 through a feeding nozzle 3 to supply the bottom block 8 therewith; forming a billet shell by cooling down the molten alloy 1 through the inner wall of the mold 5 (primary cooling-down) under the control of the level of the molten alloy 1 in the mold 5 using a float 4 such as a float distributor disposed in the mold 5; pulling the solidified billet out of the mold 5 by lowering the bottom block 8; and further cooling down the drawn billet using a refrigerant 6 (secondary cooling-down) to solidify the entire billet. As described above, according to the primary cooling-down, the molten alloy 1 is held in contact with the mold, thereby cooling down the molten alloy 1. According to the secondary cooling-down, the refrigerant 6 is driven into direct contact with the billet, thereby cooling down the billet. A shorter distance L between the primary and secondary cooling-down provides more rapid cooling-down of the aluminum alloy and the fine grain structure of the aluminum alloy. The distance L greater than 150 mm increases a time between the primary and secondary cooling-down, and consequently decreases the cooling-down of the aluminum alloy during the casting of the aluminum alloy, with a consequential failure in fine grain structure. Therefore, according to the present embodiment, the primary cooling-down was separated from the secondary cooling-down by the distance L of 150 mm or less.

The degree of the secondary cooling-down is varied with angle a at which the refrigerant 6 is urged into contact with the billet on the surface thereof. When the angle a is less than 20° or at least 80° in the casting direction 9, then the cooling water (refrigerant 6) fails to be applied perfectly to the billet surface, with a concomitant reduction in cooling capability of the cooling water. Therefore, according to the present embodiment, the refrigerant 6 was forced into contact with the billet surface at angle a between 20° or greater and less than 80°.

To further increase the cooling-down of the billet after the secondary cooling-down, a tertiary refrigerant 10 in a vessel 11 is brought into contact with the billet surface. However, the tertiary refrigerant 10 in contact with the billet at angle β of less than 20° in the casting direction 9 reduces the cooling capability of the tertiary refrigerant 10. The tertiary refrigerant 10 in contact with the billet at angle α of 80° or greater fails to permit the refrigerant 10 to flow along the billet surface. Therefore, according to the present embodiment, the tertiary refrigerant 10 was brought into contact with the billet at angle β between 20° or greater and less than 80° in the casting direction 9.

In the semi-continuous casting of the aluminum alloy, the molten alloy 1 is fed out of the mold 5 through the float 4 disposed within the mold 5 on the level of the supplied molten alloy 1, thereby controlling the level of the molten alloy 1. In the control of the level of the molten alloy 1, the float 4 ejects the molten alloy 1 within the mold 5. However, the cooling capability of the molten alloy 1 is considerably varied, depending upon angle γ at which the molten alloy 1 is ejected from the float 4. Any ejection angle γ less than 20° in the casting direction 9 causes the molten alloy 1 to heavily fall down in the casting direction 9. As a result, a solidification interface 7 in the mold 5 is located at a lower position in the longitudinal direction of the mold 5 when seen from the front of the mold 5, and consequently the molten aluminum alloy 1 is reduced in cooling-down. Any 80° or greater ejection angle γ heavily splashes the molten alloy 1 on the mold 5, resulting in poor billet appearances. Therefore, according to the present embodiment, the molten alloy 1 was ejected at angle γ between 20° or greater and less than 80° in the casting direction 9 from the float 4 that is designed to control the metal level of the molten alloy 1 supplied in the mold 5.

In general, the molten alloy is cast in accordance with the semi-continuous casting. In addition, the float is used to control the level of the molten alloy in the mold. Alternatively, the use of a hot-top casting process without the use of the float achieves increased cooling-down of the molten alloy and an increased solidification rate of the molten alloy, thereby providing fine grain structure of the alloy.

The billet grains are made fine by the addition of proper amounts of Ti and B and the selection of optimal casting conditions. There is 20 not particularly a limit to the aluminum alloy, but chief elements to be added to aluminum include Mg, Si, Mn, Zn, Cu, Fe, Cr, Ni, Li, V, Zr, Sn, Pb, Bi, Co, and Sr.

EXAMPLES

The following discusses specific embodiments of the present invention in comparison with comparative examples.

Ti and B were added to each alloy composition that is represented by corresponding one of billet compositions as illustrated in Table 1 below. Aluminum alloy billets were cast in accordance with semi-continuous casting. TABLE 1 Semi-Continuous Casting Conditions Distance between primary and TiB₂ Al_(x)- secondary Secondary Third Added Weight particle to- cooling- Float cooling- cooling- Billet (wt %) Ti-to-B size Ti_(y) down ejection down down Composition Ti B ratio (μm) ratio (mm) angle (°) angle (°) angle (°) Ex. 1 AC4C 0.15 0.03 5 5 1.2 50 40 30 40 Ex. 2 7075 0.01 0.002 5 7 1.0 45 50 40 55 Ex. 3 5182 0.008 0.001 8 4 0.8 60 30 55 40 Ex. 4 6061 0.02 0.0015 13 8 1.0 75 30 45 30 Ex. 5 2014 0.03 0.002 15 10 0.9 80 45 25 45 Ex. 6 4032 0.02 0.001 20 9 1.5 65 40 30 40 Comp. AC4C 0.08 0.0015 53 35 0.3 160 15 10 85 Ex. 1 Comp. 7075 0.02 0.4 0.5 40 0.6 140 85 30 10 Ex. 2 Comp. 2014 0.03 0.18 60 55 0.2 100 10 30 5 Ex. 3

The aluminum alloy billets were cast in accordance with the conditions as illustrated in Table 2 below. Table 2 contains results from assessments on molding capabilities during semi-solid molding and assessments on shapes of primary α (Al) phases after the semi-solid molding. TABLE 2 Assessments Molding Shape of Primary Crystal Capability α(Al) Phase after Semi- Assessment Grains during Solid Molding on billet size of Semi-Solid Spher- Fine Appearance billet Molding oidized Uniformity Ex. 1 ◯ ◯ ◯ ◯ ◯ Ex. 2 ◯ ◯ ◯ ◯ ◯ Ex. 3 ◯ ◯ ◯ ◯ ◯ Ex. 4 ◯ ◯ ◯ ◯ ◯ Ex. 5 ◯ ◯ ◯ ◯ ◯ Ex. 6 ◯ ◯ ◯ ◯ ◯ Comp. ◯ X X X X Ex. 1 Comp. X X X X X Ex. 2 Comp. ◯ X X X X Ex. 3

In the assessments on the billet appearances as illustrated in Table 2, a billet having a smoothly formed surface without defects thereon such as scoring and cold shuts was assessed as good (◯). A billet having 300 μm or less grains were evaluated as good (◯), but a billet having greater than the 300 μm grains was rated as bad (X).

In the assessments on the molding capabilities during the semi-solid molding, a billet having a good molding capability was assessed as good (◯), but a billet having a bad molding capability was rated as bad (X).

In the assessments on the shapes of the primary α (Al) phases after the semi-solid molding, a billet having spheroidized primary grains was assessed as good (◯), but a billet having insufficiently spheroidized primary grains was evaluated as bad (X). In the assessments on fine uniformities of the primary α (Al) phases after the semi-solid molding, a billet having a 500 μm or less primary a (Al) phase was evaluated as good (◯) but a billet having greater than the 500 μm primary α (Al) phase was rated as bad (X).

FIG. 1 is a photograph showing a representative example of one primary α (Al) phase having fine uniformity evaluated as good (◯). FIG. 2 is a photograph showing a representative example of another primary α (Al) phase having fine uniformity assessed as bad (X).

As described above, a semi-solid billet according to the present invention is producible through a simpler manufacturing process at lower cost than prior art semi-solid billets are.

The present invention provides a semi-solid billet having a uniformly spheroidized structure characterized by a primary a (Al) phase of 500 μm or less on average and an area proportion of 50%. As a result, the present invention provides beneficial effects in which the semi-solid billet according to the present invention finds wide applications in practice. 

1. An aluminum alloy having an improved semi-solid molding capability, comprising, in wt %, 0.005 to 0.5 Ti, 0.0001 to 0.1 B, and balance substantially aluminum.
 2. An aluminum alloy having an improved semi-solid molding capability, as defined in claim 1, wherein Ti to be added to provide fine grain structure of said aluminum alloy is added in a form of an Al—Ti master alloy before being further added in a form of an Al—Ti—B master alloy.
 3. An aluminum alloy having an improved semi-solid molding capability, as defined in claim 1, wherein amounts of Ti and B to be added to provide fine grain structure of said aluminum alloy are in a Ti-to-B-ratio of 3 to 40, (Al_(x).Ti_(y)) B_(2 (x=)1 to 7, y=1 to 9) is a particle having a size of 10 μm or less, and an atomic weight ratio of Al_(x) to Ti_(y) is 0.2 to 2.5.
 4. A method for producing a billet of an aluminum alloy having an improved semi-solid molding capability, said aluminum alloy comprising, in wt %, 0.005 to 0.5 Ti, 0.0001 to 0.1 B, and balance substantially aluminum, said method comprising: pouring said aluminum alloy in a molten state into a cylindrical, forced cooling mold from top of said mold in order to perform semi-continuous casting of said aluminum alloy; providing a billet having a solidified layer formed on a surface of said billet by executing primary cooling-down of said molten aluminum alloy using said mold; pulling said billet out of said mold by lowering a bottom block which supports a bottom end of said billet;. and feeding a refrigerant onto the surface of said billet to effect secondary cooling-down of said billet, wherein said primary cooling-down is separated from said secondary cooling-down by a distance of 150 mm or less, and wherein said refrigerant for use in said secondary cooling-down is brought into contact with said billet at an angle between 20° or greater and less than 80° in a casting direction.
 5. A method for producing a billet of an aluminum alloy having an improved semi-solid molding capability, as defined in claim 4, wherein Ti in said molten aluminum alloy is added in a form of an Al—Ti master alloy before being further added in a form of an Al—Ti—B master alloy.
 6. A method for producing a billet of an aluminum alloy having an improved semi-solid molding capability, as defined in claim 4, wherein amounts of Ti and B in said aluminum alloy are in a Ti-to-B-ratio of 3 to 40, (Al_(x).Ti_(y)) B_(2 (x=)1 to 7, y=1 to 9) is a particle having a size of 10 μm or less, and an atomic weight ratio of Al_(x) to Ti_(y) is 0.2 to 2.5.
 7. A method for producing a billet of an aluminum alloy having an improved semi-solid molding capability, as defined in claim 4, wherein after said secondary cooling-down to enhance said semi-continuous casting of said aluminum alloy, a tertiary cooling-down refrigerant is brought into contact with said billet at an angle between 20° or greater and less than 80° in the casting direction to further increase cooling-down of said billet.
 8. A method for producing a billet of an aluminum alloy having an improved semi-solid molding capability, as defined in claim 4, wherein said providing said billet having said solidified layer formed on the surface of said billet comprises expelling said molten aluminum alloy from a float at an angle between 20° or greater and less than 80° in the casting direction to feed said molten aluminum alloy out of said mold in order to control a level of said molten aluminum alloy, said float being positioned in said mold on a surface of said molten aluminum alloy. 